USB HDD sometimes very slow, sometimes not

System: Win7 Home Prem SP1, Dell 530 desktop, plenty of disk space.

My external Hitachi 1-TB USB2.0 drive (drive L: ) contains a series of Macrium Reflect image backups, each about 70-80 GB. Daily, an automatic routine runs overnight to create a new image and store it on L:. Once a week, a batch file robocopies the latest image to a separate USB2.0 portable drive (drive U: ) which then gets stored offsite. I also delete all but the last 2 images on L: and U: every week, just before the weekly batch run, to make sure there is adequate space.

The issue: about half the time lately, the batch file fails to complete the copying process in a reasonable time.
The image on those days is reported as "corrupt".

How are the drives connected to the computer?
If they are connected through a non-powered hub you could be experiencing power fluctuation problems. Try either connecting them directly to the computer or via a powered hub. Of course this is assuming that your drives are drawing their power via the USB port. It they have a separate power brick then this is not likely to be the problem. Of course you could be flooding the bandwidth of the USB 2.0 if both are connected to the same hub. HTH cheers:

@RetiredGeek:
Both are directly connected to USB ports on the main computer: one to a front port and one to a rear port. The Hitachi L: drive has its own power cord/wall wart, and the other U: drive (WD Passport) uses a short cable with proprietary mini-USB+power connector on the drive end (but experimentation shows a standard mini-USB cable will work, too).

@Sudo15:
I appreciate the suggestion and will download/run the test and post results here.

Continuing:
@Sudo15:
The website involved in running your suggestion seems to be out of order, so the test program wouldn't complete its installation.
I did run the Win7 control panel diagnostic program to check on memory performance, and it was all OK (as was the rest of the system).